This invention relates to an improved dehydrogenation process for producing indene and substituted indenes, and more particularly, to a dehydrogenation process utilizing a composite catalyst comprising cobalt oxide and molybdenum oxide. Indene is present in low concentrations (e.g. 12-16%) in ethylene or gas oil cracking coproducts, but it has been difficult to recover the indene in satisfactory yields and purity from these low concentration sources. Indene is a desirable raw material for preparing superior heat-resistant polymers.
The invention of this application is directed particularly to the preparation of indene and substituted indene from tetrahydroindene and substituted tetrahydroindene. Tetrahydroindene along with other products are formed in Diels-Alder reactions of butadiene with cyclopentadiene or its dimer, dicyclopentadiene. Substituted tetrahydroindenes are obtained when a substituted butadiene is used in the reaction. A considerable amount of research has been conducted and published on this reaction, and various suggestions have been made for optimizing the production of the various coproducts such as vinyl cyclohexane and vinyl norbornene. Attempts to separate the coproducts, and particularly, the tetrahydroindene, generally have been unsuccessful.
U.S. Pat. No. 3,183,249 describes the preparation of 5-vinyl norbornene-2 by the thermal Diels-Alder type reaction of butadiene and cyclopentadiene at a temperature of 100.degree.-400.degree. C. at pressures of from 2 to 75 atmospheres and a reaction time of 1-3 hours. The process of the patent results in conversion efficiencies to 5-vinyl norbornene-2 of about 20%. The remainder of the converted cyclopentadiene is converted into a large variety of undesirable byproducts. In U.S. Pat. No. 3,728,406, 5-vinyl norbornene-2 is prepared by reacting cyclopentadiene and/or dicyclopentadiene with butadiene, the reaction being discontinued when no more than 50% of the cyclopentadiene has been converted whereby the efficiency of converted raw material into vinyl norbornene is improved.
The dehydrogenation of indene precursers such as tetrahydroindene into indene has been described in the art and generally is conducted in the presence of dehydrogenation promoting catalysts. In U.S. Pat. No. 4,143,082, the dehydrogenation of indene precursers into indene is accomplished by contacting the indene precurser in the presence of an oxygen donor with a phosphate catalyst at elevated temperature. These catalysts, described more fully in the patent, are salts of one of the phosphoric acids. Other types of dehydrogenation catalyst have been described in the literature, and such compounds include the metal oxides, metal salts such as the halides, phosphates, sulfates, molybdates, tungstates, etc. Generally, these catalysts are characterized as compounds containing a metal having a polyoxidation state, that is, a metal having at least two oxidation states in addition to the zero state. Examples of useful polyoxidation state metals include Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Mo, Ru, etc.
In addition to the use of polyoxidative state metals, oxidation catalysts also may be combined with one or more monooxidation state metals which act as promoters, initiators, stabilizers and the like. The single oxidation state metal or metal compounds include the alkali metals, and polyvalent metals such as magnesium, aluminum, calcium, scandium, zinc, etc. The use of cobalt and molybdenum oxides promoted with potassium oxide in dehydrogenating indane to indene is reported in Czech Patent No. 135,251. The catalyst bed contained 3% CoO, 10% MoO.sub.3 and 0.3% K.sub.2 O. A review of the various catalysts useful in oxidative dehydrogenation of organic compounds is found in U.S. Pat. No. 3,925,498.